<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>

        div:empty {
            border: 1px solid black;
            height: 100px;
            /* 有空格都不行，但是注释竟然不算内容 */
        }


    </style>
</head>
<body>
    <div><span></span></div>
    <div></div>
    <div> </div>
    <div>456</div>
    <div><!--  --></div>
</body>
</html>